This invention relates to an apparatus for positioning a carrier means, and more specifically, to a system for controlling the positioning of a carrier means along a line of print in either of two directions in a rapid, efficient, and error-free manner in order that the carrier means is positioned at a desired destination position as smoothly and efficiently as possible.
The prior art teaches many systems for positioning a carrier means along the line of print in either a forward or a reverse direction. Many of these systems employ a motor to drive the carrier and many utilize some type of motor control system for controlling the operation of the motor. Most systems of the prior art, however, employ a tachometer which is coupled to the drive shaft of the motor, and the tachometer is used to feed back an analog signal which is proportional to the speed of the motor. This signal is then compared with some type of reference signal in a comparator or the like and is used to control the operating speed of the motor by any number of means known in the art. The present invention employs an electronic tachometer which is much cheaper, more efficient and easier to maintain than the prior art tachometers.
The carrier positioning systems of the prior art frequently operated to drive the carrier means at a constant speed until it arrived at a destination position, at which time it would be driven against a destination stop with wasted torque, wear and tear on the motor and associated stop apparatus, and with a great amount of noise. Some systems of the prior art would operate to drive the carrier at an ever increasing torque until a point midway between the original position and the destination position was reached, and at that time, would reverse the direction of current through the motor so as to slow the carrier as it approaches its destination position. Such systems do not employ any real speed control since the speed is either continually increasing or continually decreasing and a constant speed is never maintained. Many such systems are susceptible to overshoot or undershoot or must employ the stop mechanisms mentioned above with their associated disadvantages.
The system of the present invention overcomes the various disadvantages of the prior art by providing that the carrier means can be driven at either a predetermined high speed state which is entered whenever the carrier is more than a predetermined number of carrier positions from its destination position and at a predetermined low speed state which is entered when the number of carrier positions are less than or equal to said predetermined number of carrier positions from its destination position. A state machine with associated logic is used to insure a smooth and efficient transition when the carrier is positioned from a stop to the predetermined high speed state and then to the predetermined low speed state prior to being again stopped at the new desired destination position. An electronic tachometer speed control means is used to maintain a relatively constant predetermined high speed while in said high speed state and to maintain a relatively constant predetermined low speed while in said low speed state.